Gravitational waves and massive black holes? The LISA and LISA pathfinder missions

Though they have not been directly detected so far, gravitational waves are a necessary consequence of Einstein's theory of General Relativity. They distort space and time, changing the perceived distances between free macroscopic bodies. A gravitational wave passing through the Solar System creates a time-varying strain in space that periodically changes the distances between all bodies in the Solar System in a direction that is perpendicular to that of wave propagation. These could be the distances between spacecraft and the Earth, as in the case of Ulysses or Cassini (attempts were and will be made to measure these distance fluctuations), or the distances between shielded 'proof masses' inside spacecraft that are separated by a large distance, as in the case of USA. The collaborative NASA/ESA Laser Interferometer Space Antenna (LISA) mission, planned for launch in 2012, will be the first space mission to search for these elusive gravitational waves. As the technology needed for the project is highly demanding, a precursor technology-demonstration mission is considered to be a necessary pre-requisite. Called LISA Pathfinder (formerly SMART-2) and planned for launch in 2008, it was given the go-ahead on 7 June by ESA's Science Programme Committee (SPC).